In general, autoclaved aerated concrete (“ordinary AAC”) is one example of lightweight precast concrete which is formed under a high temperature and high pressure (for example, 190° C., 12 atm) cured for 6 to 8 hours using raw materials such as calcareous materials of cement and lime (CaO), siliceous materials such as silica (SiO2), silica sand (SiO2), and other materials such as gypsum (CaSO4), recycled materials produced in manufacturing such as fly ash, metal aluminum and other aerating agents, surfactants for stabilizing the bubbles, and other fillers. The aerating agent causes air voids to form in the matrix and increases the porosity of the material. This causes an increase in the volume and thereby reduces the density of the material.
Ordinary AAC products offer a number of advantages over conventional concretes such as good strength-to-weight ratio, resistance to fire, corrosion, termites and molds, as well as good thermal insulation and sound deadening properties. Due to their lightweight and dimensional accuracy, ordinary AAC products can be assembled with minimal waste thereby reducing the need for additional equipment in construction and assembling. They offer high durability and require minimum maintenance. The lightweight of an ordinary AAC also helps with lowering shipping costs. Although the compressive strength of an ordinary AAC depends on its total void volume, commercially available ordinary AAC achieve about 5 N/mm2 at an absolute dry density of 0.50 g/cm3. These properties sufficiently meet the strength requirements for building materials.
Despite their overarching benefits, ordinary AAC are prepared by processes that commonly suffer from a number of deficiencies. The manufacturing process of ordinary AAC involves special equipment, large energy consumption, and excessive carbon dioxide emission, leaving unfavorable carbon footprint. Ordinary AAC are typically cured in autoclaves at temperatures ranging from 150° C. to 190° C. and at pressures ranging from 0.8 MPa to 1.2 MPa. These conditions lead to the creation of a stable form of tobermorite, which is the primary bonding element in ordinary AAC. In addition, they are relatively expensive due to high finishing costs and are also difficult to recycle.
As building materials for making walls, room partitions, and floors, ordinary AAC panels use either reinforcing structures (for example, iron rods) embedded inside them or non-reinforcing structures. Such ordinary AAC also consist of large number of pores and bubbles that can simultaneously hold some amount of water. This water is found to be present even when the ordinary AAC is in a usual usage environment. Since ordinary AAC have a large number of air bubbles inside them, carbon dioxide from the air can infiltrate inside the ordinary AAC over time. The infiltrated carbon dioxide can also dissolve into such water, where calcium derived from various components is also present.
Further, the water present in the ordinary AAC also helps in the reaction between calcium and carbon dioxide to form calcium carbonate as a precipitate. This is generally referred to as “carbonation”. This carbonation phenomenon usually occurs in conventional concrete materials. As a result of carbonation, the concrete structure becomes dense and its strength increases, while its water adsorption falls. Other phenomena simultaneously occur, for example, shrinkage of the structure (matrix), formation of micro-cracks, and drop in strength due to rusting of the iron reinforcement accompanying neutralization. Similar phenomena can arise with ordinary AAC. In ordinary AAC, when such carbonation proceeds excessively over a long period of time, the above-mentioned shrinkage of the matrix can cause problems accompanying carbonation in the same way as in the case in conventional concrete materials.
To suppress carbonation, a hydrothermal reaction is employed in the production of ordinary AAC by steam curing under a high temperature and high pressure (“autoclaving”) to cause formation of a sufficient amount of the mineral crystal, tobermorite and thereby decreasing the speed of carbonation and suppressing the issues caused by carbonation, which occurs over a long period of time in an ordinary usage environment.
For example, Japanese Patent Publ. No. 5-310480A describes ordinary AAC as a structure where air bubbles are connected to form innumerable pores that extend from the surface to the inside, allowing water to be easily absorbed from the surface. Since the absorbed water contains dissolved carbon dioxide gas, it reacts with the tobermorite crystals and CSH gel in the ordinary AAC to form calcium carbonate and cause the so-called carbonation phenomenon. Japanese Patent Publ. No. 5-310480A also describes the general practice of making ordinary AAC panels that include cage-like iron reinforcement or steel netting or other reinforcement material. When iron reinforcement or other reinforcement is used in case of ordinary AAC it has a tendency to absorb water right to its center, necessitating rust-prevention.
While the above method may be applied to decrease the rate of carbonation when using ordinary AAC in an ordinary usage environment, there are situations where the amount of formation of tobermorite at the time of production of ordinary AAC is small or when ordinary AAC is used in an environment, different from an ordinary usage environment, where the concentration of carbon dioxide in the air is high. In such circumstances even if these methods are used, excessive carbonation can still become a serious problem. Therefore, there is a need to mitigate the problems that may occur due to the excessive rate of carbonation in case of ordinary AAC.
Recently, to avoid in principle the problems such as carbonation in concretes and ordinary AAC, WO2012/122031A discloses an improved bonding matrix in place of conventional cement, concrete, or other ceramic material such as CaO.2SiO2.4H2O and CaO.H2O or other weak hydrated Portland cement. The bonding element of such a bonding matrix is, for example, comprised of a precursor particle comprised of calcium silicate (CaSiO3). This precursor particle can react with the carbon dioxide dissolved in water. Calcium cations are leached from calcium silicate particles and transform the peripheral portion of the calcium silicate particle core into calcium-deficient. As the calcium cations continue to be leached from the peripheral portion of the core, the structure of the peripheral portion eventually become unstable and breaks down, thereby transforming the calcium-deficient peripheral portion of the core into a predominantly silica-rich first layer. Meanwhile, a predominantly calcium carbonate second layer precipitates from the water. The formation of these layers is not uniform in the case of composite particle.
Specifically, the first layer and the second layer are formed from the precursor particle by a reaction of H2O+CaSiO3+CO2═CaCO3+SiO2+H2O. That is, carbon dioxide selectively reacts with the Ca cations of the silica precursor core whereby the peripheral portion of the precursor core is transformed to a silica-rich first layer and calcium carbonate-rich second layer. The presence of the first layer and the second layer on the core acts as a barrier to further reaction of the calcium silicate particles and carbon dioxide. As a result, a bonding element comprising a core, first layer, and second layer is formed.
More particularly, the bonding element described in WO2012/122031A is already sufficiently carbonated at the time of production, so at least the problems of carbonation that occur along with the elapse of time in the concretes and ordinary AAC of the prior art, are avoided. This bonding element can be formed by the method of gas-assisted hydrothermal liquid phase sintering. In such a method, a porous solid body including a plurality of precursor particles is exposed to a solvent, which partially saturates the pores of the porous solid body, i.e., that the volume of the pores are partially filled with water. A gas comprising a reactant of carbon dioxide is introduced into the partially saturated pores of the porous solid body where the solvent dissolves the reactant. The dissolved reactant is depleted from the solvent due to the reaction, but the gas comprising the reactant continues to be introduced into the partially saturated pores to supply additional reactant to the solvent.
As the reaction between the reactant and the at least first chemical element of the precursor particles progresses, the peripheral portion of the precursor particle is transformed into the first layer and the second layer. The presence of the first layer at the periphery of the core eventually hinders further reaction by separating the reactant and the at least first chemical element of the precursor particle, thereby causing the reaction to effectively stop, leaving a bonding element having the core as the unreacted center of the precursor particle, the first layer at a periphery of the core, and a second layer on the first layer. As a result of the transformation, the core has a shape similar to the precursor particle, but has a smaller size. The first layer and the second layer partially or completely cover the core and have uniform or non-uniform thicknesses which enable formation of porous structures depending on the size and shape of the pores which surrounded the precursor particle during the transformation process. The resulting bonding element includes the core, the first layer and the second layer, and is generally larger in size than the precursor particle, filling in the surrounding porous regions of the porous solid body and possibly bonding with adjacent materials in the porous solid body. As a result, the net-shape of the products that may be formed have more or less the same size and shape as their original forms but a higher density than the porous solid body.
Furthermore, WO2014/165252A discloses a carbonation-cured material constituted by an aerated composite material using a carbonatable calcium silicate composition and a process of production of the same. As explained above, ordinary AAC utilizes the hydrothermal reaction due to autoclaving at the time of production so as to form tobermorite crystals and cure the material, followed by a reduction in temperature and pressure to respectively ordinary temperature and ordinary pressure. The material is then taken out from the autoclave for processing its surfaces and end-parts as per the product specifications before supplying it for practical use.
In the process of producing aerated composite material using a carbonatable calcium silicate composition (“carbonation cured ACC”), the carbonation occurs when the calcium and carbon dioxide are reacted. This novel method of replacing conventional Portland cement for producing AAC can significantly reduce energy requirement and CO2 emissions. The disclosed carbonatable calcium silicate compositions are made from widely available, low-cost raw materials by a process suitable for large-scale production with flexible equipment and production requirements. This unique approach is also accompanied by a remarkable proficiency for permanently and safely sequestrating CO2. A wide variety of applications can benefit from the invention, from construction, pavements and landscaping, to infrastructure and transportation through improved energy consumption and more desirable carbon footprint.
Thus, in an aerated composite material made from a carbonatable calcium silicate composition, the high temperature, high-pressure atmosphere in ordinary AAC is not required and autoclaving becomes unnecessary. It becomes possible to significantly lower the temperature at the time of curing. That is, in an aerated composite material using a carbonatable calcium silicate composition, carbonation is utilized for the curing itself, so the carbonation after production can be greatly reduced and the occurrence of problems in case of ordinary AAC associated with excessive carbonation can be fundamentally eliminated.
WO2014/165252A describes an aerated composite material made from calcium silicate compositions where a plurality of voids comprise bubble-shaped and/or interconnected channels account for 50 vol % to 80 vol % of the composite material and where the composite material exhibits a density of approximately 300 kg/m3 to 1500 kg/m3, exhibits a compressive strength of approximately 2.0 MPa to approximately 8.5 MPa (N/mm2), and exhibits a flexural strength of approximately 0.4 MPa to approximately 1.7 MPa.
However, the compressive strength of an aerated composite material depends on the density and further the density depends on the void volume. The void volume can more particularly be divided into the bubble volume and the pore volume. The bubble volume depends on the amount of addition of the foaming agent (aerating agent) such as metal aluminum (aluminum powder). Changing the amount of addition of this foaming agent can easily control the bubble volume. On the other hand, the pore volume can be controlled by the water content present at the time of mixing of the raw materials (water/solids (W/S) ratio) and the degree of advance of carbonation at the time of curing. That is, in principle, these factors can be changed to control the density-strength property. The literature, however, does not specifically disclose or teach at all what kind of compressive strength can be achieved at a specific void volume and a specific density much less specifically disclose, teach, or suggest the void volume and more particularly the bubble volume and pore volume.